Add support for floating point immediates to MC instruction printing. ARM

Add support for floating point immediates to MC instruction printing. ARM
VFP instructions use it for loading some constants, so implement that
handling.

Not thrilled with adding a member to MCOperand, but not sure there's much of
a better option that's not pretty fragile (like putting a double in the
union instead and just assuming that's good enough). Suggestions welcome...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113996 91177308-0d34-0410-b5e6-96231b3b80d8